Automatic stepped transmissions for automotive use customarily employ hydraulic fluid which controls the engagement of operating elements within the transmission, e.g. by tightening friction bands or the like. The pressure of the operating fluid, hereinafter and for simplicity called the transmission fluid, can be controlled in dependence on various operating parameters. Under most customary operating conditions, the pressure is set to depend on loading of the engine; it has also been proposed to include vehicle speed as an operating parameter.
Pressure of the operating hydraulic fluid can be controlled also in another manner, for example by introducing a parameter representative of engine speed as well as drive speed, or input speed of a shaft to the transmission. The referenced German Patent Disclosure Document DE-OS No. 28 43 183 describes such a system. In addition, it is known to change the operating pressure of the friction band in the transmissions in dependence on other parameters, for example on developed drive torque. This, however, requires extensive and complex sensing apparatus.
The various systems which have been proposed require sensors which, at times, are difficult to fit into an automotive vehicle, or sensors which do not provide output signals which unambiguously characterize the required parameters which are desirable to control the operating pressure of the hydraulic fluid in an automatic transmission.